Stable high temperature coating

ABSTRACT

A high temperature metallic threaded fastener such as a stainless steel or titanium nut or externally threaded fastener, is coated with a high temperature lubricating coating. The coating is an aqueous solution or suspension of titanium silicate and molybdenum disulfide, and may also include a wetting agent to maintain materials in suspension. Following baking at an elevated temperature the dried lubricating coating includes at least 10% silicon and 20% molybdenum. The coating provides lubrication and avoids seizing or galling despite cycling through very high temperatures well above 500° C. (about 932° F.).

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is based upon provisional application Ser. No. 60/523,348 filed Nov. 18, 2003.

FIELD OF THE INVENTION

This invention relates to high temperature fasteners, including high temperature, lubrication coatings for nuts, bolts and studs, and other metal parts requiring high temperature lubrication coatings.

BACKGROUND OF THE INVENTION

For use at high temperatures, for example in applications where threaded nuts are employed to secure parts together in or adjacent to aircraft turbines, it has been customary to use a coating of silver on nuts formed of stainless steel, titanium or other high temperature compatible metals. However, at the very high temperatures which are encountered silver coated nuts may be subject to corrosion, and possible failure of the nut and threaded stud or bolt assembly.

SUMMARY OF THE INVENTION

Accordingly a principal object of the invention is to provide an improved high temperature coating for threaded fasteners such as nuts, bolts and threaded studs; as well as for other metal parts requiring high temperature lubrication.

The coating is preferably formed with silicon and molybdenum as the principal components of the coating, and with at least 10% silicon and 20 percent molybdenum, and preferably more than 20% silicon and more than 40 percent molybdenum.

In accordance with one illustrative embodiment of the invention, nuts having a lubricating coating as identified above are employed to hold parts together, in a very high temperature environment such as in proximity to a turbine engine or the like, with the coating serving to provide lubrication for the thread surfaces as the nuts are removed and later replaced.

One aspect of the invention, therefore, involves the combination of a source of high temperature, such as a turbine or jet engine, and threaded fasteners employed to hold components together in the immediate vicinity of the source of high temperatures, and with the threaded fasteners being coated with the dry lubricant including at least 10% silicon and at least 20% molybdenum.

Further, the temperatures of the high temperature source may be above 1,000° F. (about 538° C.). In addition, the coating may include 25% or more of silicon and 40% or more of molybdenum.

Concerning the process for coating threaded fasteners, such as nuts, bolts and studs, the coating liquid or suspension may be water based and may contain silicon and molybdenum in the form of certain compounds such as liquid sodium silicate and molybdenum disulfide, and a very small quantity of a wetting agent to maintain materials in suspension. The liquid coating may be applied to nuts or other threaded fasteners by spraying and/or immersing the threaded fasteners in the liquid. The coating is subsequently basked onto the fastener at an elevated temperature for a substantial period of time until the coating is dried onto the fastener.

The resultant coating is about 0.0002 to 0.0006 inch thick.

Following the foregoing treatment the composition of the coating is primarily silicon and molybdenum with the sodium and sulfur having been driven off in the baking process. In one analysis, the resultant coating was determined to be about 25% to 35% silicon and about 45% to 55% molybdenum, with traces of other materials.

One major manufacturer of aircraft engines has stated that “Due to the potential problems associated with the use of silver when in contact with titanium and heat resisting alloys at elevated temperatures, the search for a replacement coating has become a priority”. A further report indicated that the coating as described in this specification “has provided test results similar to a silver coated locking nut”, but without the corrosion associated with silver coatings.

An advantage of the invention is that it provides a lubricating coating for metal parts operative up to about 1400° F. as compared with prior lubricating coatings which were only marginally effective about 1200° F.

Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and from the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are three views of a high temperature metal nut coated with the coating illustrating the principles of the invention; and

FIG. 4 is a schematic showing of a high temperature environment with which the coated nuts of FIGS. 1-3 may be used.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

While the specification describes particular embodiments of the present invention, those of ordinary skill can devise variations of the present invention without departing from the inventive concept.

Referring more particularly to the drawings, FIGS. 1-3 represent a high temperature nut 12 preferably formed of stainless steel or titanium, for examples. The nut 12 is coated with a high temperatures lubricating coating as discussed in greater detail elsewhere in this specification.

FIG. 4 is a schematic showing of an apparatus 14 such as a turbine or other product operating at very high temperatures. The apparatus 14 has outwardly extending flanges 16. Studs or bolts 20 are secured to the plate 22; and the nuts 12 have internal threads which mate with the threads on the studs or bolts 20. When the nuts are tightened, the apparatus 14 is firmly secured to the plate 22.

The nuts 12 and/or the studs or bolts 20 are preferably coated with a coating having silicon and molybdenum as their principal components.

One batch of coating material which has been successfully used included the following: Weight % Sodium Silicate 143 g 21% Molybdenum disulfide  75 g 11% Wetting agent  1 g Less than  1% Water 454 g 67% Total 673 g 100% 

The solution was sprayed onto nuts, and the nuts were immersed in the solution. The nuts were subsequently based at 600° F. to 1000° F. (about 316° C. to 538° C.) for one hour. Subsequently, users had successful results in that the nuts did not seize or gall with cycling to elevated temperatures and back to room temperature.

Concerning the materials, the liquid sodium silicate is a milky fluid, and has a chemical formula of Na₂0 X SiO₂, where X=2 and higher. It is available from Orbital Chemical Company on Katella Avenue in Los Alamitos, Calif. 90702. The name of the manufacturer is Diamond Shamrock Chemicals Co., 351 Phelps Ct., P.O. Box 152,300, Irving, Tex. 75015-2300. The boiling point is 214° to 216° F. (about 101° C.).

Concerning the molybdenum disulfide, it is an odorless, black, lustrous, finely divided powder. The molecular formula is MoS₂. The melting point is about 599° F. (about 315° C.). It is available from Climas Molybdenum Marketing Corp., 1501 W. Fountainhead Parkway, Tempe, Ariz. 85285-2015.

Incidentally, the melting point of molybdenum is about 2620° C. (about 4684° F.) and the melting point of silicon is about 1420° C. (about 2524° F.).

The wetting agent which was used by Trycol 5952 available from Parker Amchem, a Henkel Corporation. The wetting agent is helpful in maintaining the other ingredients in suspension as the coating is applied to the threaded fasteners. In order to thoroughly coat the inner threads of nuts, the nuts may be both immersed in the dispersion and the liquid dispersion may be sprayed onto the nuts or other threaded fasteners.

In one analysis of the coating it was reported that the coating was composed of the following by weight: Silicon 25-35%  Molybdenum 45-55%  Nickel 4-9% Chromium 3-4% Cobalt 2-3% Aluminum 1-2%

It is noted that the major components are silicon and molybdenum, with the sodium and sulfur in the original compounds having been dispersed as a result of the high baking temperature. The remaining materials may be present from samples which may inadvertently have been scraped off of some of the material of the threaded fastener along with the coating.

It is noted in passing that the assembly of the coated fasteners may also include the use of lubrication such as Graphite petrolatum (A507-507-F201) or lubricating oil (MIL-L-7808) or other assembly aids.

In conclusion, in the foregoing detailed description one specific illustrative embodiment of the invention has been disclosed. However, it is to be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. Thus, by way of example and not of limitation, other silicon and molybdenum materials may be employed in the coating to be applied to the threaded fasteners. In addition other wetting agents and solvents, or carriers may be used. Further, the proportions of the input ingredients may be varied over a substantial range. Also, in addition to threaded fasteners the coating is applicable to other high temperature applications where parts are in engagement but must be moved relative to one-another. Accordingly, the present invention is not limited to the specifics as set forth in the present specification. 

1. A method comprising: coating a high strength internally threaded nut with a coating of sodium silicate and molybdenum disulfide and a wetting agent in a water suspension; baking the coated nut at a temperature above 500° F. (about 260° C.) for at least one-half hour, until the coating is dry, and has a composition of at least 10% silicon and at least 20% molybdenum, mounting the coated nut onto an externally threaded bolt or stud adjacent to a source of high temperatures above 500° C. (about 932° F.); periodically raising the temperature of the nut to a temperature above 500° C. (about 932° F.); periodically lowering the temperature to a temperature less than 100° C. about 212° F.); and removing the nut from the bolt or stud and later replacing it onto the stud or bolt.
 2. A method as defined in claim 1 wherein said suspension includes at least 5% by weight of molybdenum sulfide and at least 10% by weight of sodium silicate.
 3. A method as defined in claim 1 wherein the nuts following coating are baked until the resultant dried coating which is formed has a composition of at least 20% silicon and at least 35% molybdenum.
 4. A method as defined in claim 1 wherein the nuts are baked at a temperature between 600° F. and 1000° F. (about 316° C. to 538° C.).
 5. A method comprising: treating a high strength metal threaded fastener to produce a threaded fastener having a coating including at least 20% silicon and at least 35% molybdenum; mounting the threaded fastener to a mating threaded fastener adjacent to a source of high temperature above 500° C.; periodically raising the temperature of the threaded fastener to a temperature above 500° C.; periodically lowering the temperature to a temperature less than 100° C. (212° F.); and removing the threaded fastener from the mating threaded fastener and subsequently replacing it.
 6. A method as defined in claim 5 wherein the coated threaded fastener is a nut.
 7. A method as defined in claim 5 wherein the coated threaded fastener is an externally threaded fastener.
 8. A method as defined in claim 5 wherein the fasteners are formulated and baked until the resultant dried coating which is formed has a composition of at least 20% silicon and 35% molybdenum.
 9. A high temperature resistant threaded fastener comprising: a threaded fastener; a baked coating on said threaded fastener including at least 10% silicon and at least 20% molybdenum.
 10. A threaded fastener as defined in claim 9 wherein said coating includes at least 20% silicon and at least 35% molybdenum.
 11. A fastener as defined in claim 9 wherein said fastener is a nut.
 12. A fastener as defined in claim 9 wherein said fastener is externally threaded.
 13. A fastener as defined in claim 9 wherein said fastener is formed of stainless steel.
 14. A fastener as defined in claim 9 wherein said fastener is formed of titanium.
 15. A method comprising: coating a high strength internally threaded metallic unit with a coating of sodium silicate and molybdenum disulfide and a wetting agent in a suspension; and baking the coating nut until the coating is dry, and has a composition of at least 10% silicon and at least 20% molybdenum.
 16. A method as defined in claim 15 wherein said internally threaded metallic unit is a nut.
 17. A high temperature resistant lubricated part comprising: a metal part; a baked coating on said metal part including at least 10% silicon and at least 20% molybdenum. 